docs: def then_by_descending(self, func):
          """
          Subsequent sorting function in descending order
          :param func: lambda function for secondary sort key
          :return: SortedEnumerable instance
          """
          if func is None:
              raise NullArgumentError(
                  u"then_by_descending requires a lambda function arg")
          self._key_funcs.append(OrderingDirection(key=func, reverse=True))
          return SortedEnumerable(self._key_funcs, self._data)